The present invention relates to a chuck for holding a substrate in a chamber.
In semiconductor fabrication, a chuck is used to hold a substrate in a chamber for processing of a layer on the substrate. A vacuum chuck holds a substrate by evacuating gas from below the substrate through a vacuum port; a mechanical chuck has clamps that grip the peripheral edge of the substrate; and an electrostatic chuck holds a substrate by electrostatic attractive forces. While different types of chucks can be used to securely hold a substrate, electrostatic chucks are generally preferred because they press the substrate down more uniformly across a surface of an underlying support, thereby providing more uniform heat transfer rates across the back of the substrate; they allow use of a larger area of the substrate; and they are less prone to form contaminant particles that deposit on the substrate surface. A typical electrostatic chuck comprises an electrode covered by a dielectric, such as ceramic or polymer. When the electrode is electrically charged, electrostatic charges accumulate in the electrode and substrate, and the resultant electrostatic force holds the substrate on the chuck. Once a substrate is securely held on the chuck, process gas is introduced into the chamber and a plasma is formed to process the substrate.
Typically, the temperature of the substrate is controlled by holding helium gas behind the substrate to enhance heat transfer rates across the microscopic gaps at the interface between the back of the substrate and the surface of the chuck.
However, when the microscopic gaps are very small, variations in the heights of the gaps reduce the flow of helium gas to blocked-off regions behind the substrate. This results in an uncontrolled pressure gradient of helium gas across the back of the substrate which can cause variations in heat transfer rates across the substrate. In one solution, commonly assigned U.S. Pat. No. 5,748,435 to Parkhe describes a chuck having a gas inlet and a plurality of gas exhaust ports terminating at its surface. The pressure variation of helium gas across the back of the substrate is reduced because helium gas provided through a gas inlet diffuses toward more than one exhaust port, thereby more uniformly spreading the helium gas across the entire back surface of the substrate. This provides more uniform heat transfer rates across the entire substrate.
However, equalizing the pressure of helium gas across the back of the substrate does not always equalize temperatures across the processing surface of the substrate, especially for processes where different parts of the substrate surface are exposed to different heat loads from the plasma or from the chamber. For example, a substrate is sometimes subjected to non-uniform heat loads which give rise to concentric bands having different temperatures across the processing surface of the substrate. These bands of different temperatures can occur due to the non-uniform coupling of energy from the RF energizing electrode below the substrate to the plasma sheath which results in different rates of bombardment or kinetic energy of the energetic plasma species onto the surface of the substrate. Non-uniform heat loads also arise from varying levels of surface radiation emitted by the chamber components or chamber walls.
Accordingly, it is desirable to reduce the variations in temperature across the substrate surface, especially during processing of the substrate. It is also desirable to control temperatures at different regions across the processing surface of the substrate to compensate for the non-uniform heat loads seen by the substrate during processing. There is also a need for a method for maintaining uniform temperatures across the substrate surface.